To perform an analysis representative of a laser beam, a sample of this beam is taken (=the beam is sampled).
It will be recalled that a high-energy and large size pulsed laser beam is obtained by means of a CPA device, CPA being the acronym for “Chirped Pulse Amplifier” that can be seen in FIG. 1 and which comprises, at the input, a stretcher 1 capable of stretching a low-energy laser pulse as a function of the wavelength, linked to an amplifier 2 capable of amplifying the stretched pulse into a high-energy stretched pulse, and linked to a vacuum compressor 3 capable of compressing the stretched and amplified pulse. At the output of the compressor, a high-energy and large size laser pulse is obtained, transmitted in a vacuum for energies greater than 4 TW. In the lasers of TW, even multi-PW, class, the diameter of the laser pulse at the compressor output is of centimetric, even metric, class.
To perform the space-time characterization of such a high-energy system, it is necessary to sample only a very small part of the beam so as not to damage the analysis device, and to reduce its size in order to adapt it to that of this analysis device while retaining its properties.
It is known practice to sample such a beam using:                a so-called “leaky” mirror 32, shown in FIG. 2, placed in the vacuum enclosure 31 of the compressor, but at the output of the compression elements and upstream of an output window 33, and which exhibits a transmission less than 2% to take only a small sample of the compressed main beam,        a reducing afocal 42 corrected of aberrations, situated downstream of the output window of the compressor, and        a device 43 for measuring the reduced sampled beam.        
It will be recalled that the upstream downstream direction is that of the propagation of the laser beam.
However, such a sampling device presents a number of drawbacks:                The sampled beam has passed through the leaky mirror 32 before being measured. In the case of a measurement of ultra-short pulse duration, it is necessary to ensure that the optical path traveled by each wavelength (i.e. the spectral phase) which constitutes the pulse is the same on the main beam and the sampled beam. It is also essential to faithfully retain the spectral distribution of the energy (i.e. the spectral intensity). It is therefore essential to compensate this travel over the optical path of the compressed main beam which itself has been reflected and has not therefore passed through the material of the mirror 32. However, such a compensator is difficult to produce given the energy of the main beam at the compressor output. In effect, such a beam would damage any material to be passed through.        Also, the sampled beam undergoes distortions from the leaky mirror, which are also difficult to compensate on the main beam.        Furthermore, a very small transmission (i.e. <2%) requires a very good uniformity of the sample-taking over the entire pupil of the mirror and over the entire spectral band of the mirror. Obtaining transmissions less than 2% with variations less than 0.2% is very difficult to achieve for the leaky mirror manufacturers.        
Consequently, there remains, to this day, a need for a system that makes it possible, for these high-energy and large size beams, to perform a sampling without transforming the space-time properties thereof.